Minimizing UPS and Gen-Set Incompatibility

When it comes to standby power systems, it's worth the extra time and effort it takes to work out compatibility issues upfront. To protect critical loads against outages of longer duration, a battery-based UPS is often backed up with a standby engine gen-set, which may be sized to handle just the UPS or to provide power to additional loads. There's no doubt about it. In today's fast-paced information

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When it comes to standby power systems, it's worth the extra time and effort it takes to work out compatibility issues upfront.

To protect critical loads against outages of longer duration, a battery-based UPS is often backed up with a standby engine gen-set, which may be sized to handle just the UPS or to provide power to additional loads.

There's no doubt about it. In today's fast-paced information age, reliable power is getting more important by the day. And as Internet business, e-commerce, and the recent data center boom continue to increase the demand for round-the-clock power, the term “downtime” takes on a whole new meaning. So when utility power fails, instantaneous backup to critical systems better be ready. Unfortunately, this is easier said than done.

How do you ensure reliability of on-site power systems? In many installations, batteries in the UPS provide power for about 10 min to 15 min after a power failure. To protect critical loads against outages of longer duration, a battery-based UPS is often backed up with a standby engine gen-set, which may be sized to handle just the UPS or to provide power to additional loads.

Unfortunately, a history of compatibility issues exists between UPS systems and gen-sets. When gen-set and UPS misoperation create a loss of power to the critical load, it is natural to assume something must be wrong with one or the other. But compatibility conflicts can occur when both systems operate as designed. Although there may be no easy solution to this problem, there are steps you can take to achieve better system reliability.

What are the issues?

The most pronounced compatibility problems occur on small (under 100kW) gen-sets and gaseous-fueled gen-sets — when the UPS is the only load served, or when it is the largest single load on the system. In the past, the most common solution involved oversizing the gen-set by two to five times. In addition to the fact that this approach is very expensive, it also does not guarantee compatibility. This can lead to operating and service problems with the gen-set due to under-loading of the engine.

Here are some typical problems reported in UPS/gen-set compatibility situations in the field:

Failure of the UPS to acquire and accept power from the gen-set,

Instability of the gen-set frequency or voltage when powering the UPS,

Inability of the UPS to synchronize its output to the gen-set,

Inability to enable the operation of the maintenance bypass circuit,

Excessive output voltage distortion, and

Power transfer issues.

When problems occur, where do you turn for answers? There are some main sources of instability that commonly threaten the reliability of any on-site power system. The following list should help you troubleshoot power failures more effectively.

Misadjustments. Manufacturers usually design, ship, and set up gen-sets for the fastest transient performance possible while maintaining voltage and frequency stability. However, a UPS system often demands a higher level of stability than a linear load and may operate better with what the gen-set supplier might perceive as sluggish performance. A gen-set might also have subtle hardware problems that allow the machine to operate successfully on linear loads but not on the more challenging UPS load.

Any problems that stem from misadjustments may be difficult to detect once you install the gen-set because sufficient monitoring equipment is typically unavailable to document these conditions. On-site testing with a load bank is expensive and time-consuming, but is generally the only way to determine if a system is operating correctly. Ideally, you should test gen-sets used in UPS applications at the factory at rated load and power factor. Tests should include transient load testing at various load levels (with verification of voltage dip and recovery times) to observe system stability and transient performance. In this case, absolute values are not as important as verifying stability of the system.

Voltage-regulator sensitivity. A gen-set uses an automatic voltage regulator (AVR) to monitor output voltage and control field strength to maintain a constant output voltage under varying load conditions. The type of regulator you use can make a significant difference when powering UPS systems.

For example, AVRs that use silicon-controlled rectifiers (SCRs) to switch excitation power on and off can be problematic when used with UPS systems. When gen-sets supply nonlinear loads, voltage waveform “notching” induced by rectifier switching in the UPS causes the AVR to switch off the excitation current at the wrong time. This, in turn, reduces the gen-set output voltage, which causes the AVR to overcompensate for the output voltage decrease and sets up an oscillation that can cause instability or system failure. An early solution to this problem was to apply filters or isolation transformers to the AVR sensing input to prevent voltage distortion from reaching the AVR, but these solutions often prevent the regulator from quickly responding to real KVA demands in large motor-starting applications.

Because of the limitations of filtering AVR inputs, many gen-set manufacturers have turned to high-speed voltage regulators, which provide pulse-width-modulated output to the exciter of the alternator. By providing excitation power in short bursts rather than depending on commutation of the AC voltage waveform to switch off the excitation power, the system remains unaffected by waveform notching. Waveform notching can also cause instability by disrupting the power supply for the AVR. You can overcome this by adding a separate isolated power supply to the excitation system, such as a permanent magnet generator.

Control-loop compatibility. When an SCR-controlled device (like the rectifier in a UPS) operates, it depends on precise timing of the firing of the SCRs to accurately control output power from the rectifier. This firing time is critical because the SCR will turn on at a specific time, but the “off” command only occurs when the AC power approaches zero.

Inappropriate adjustments. Standby gen-sets are normally designed to respond very quickly to transient changes in load. However, when powering a UPS, fast response to transient loads is counterproductive. In fact, many older UPS systems are very tolerant of voltage and frequency excursions, but very intolerant of fast excursions. Detuning the gen-set is a potential fix to this problem. However, keep in mind that adjustment of gen-set transient performance may impact the ability of the unit to supply other types of loads on the system such as large motor loads. This problem may be most easily addressed in the SCR firing control logic of the UPS. In fact, many new UPS systems now include control logic that compensates for changes in input power frequency, which makes them immune to input frequency fluctuations.

Inability to synchronize. With a gen-set, the frequency of the output voltage is not constant. It varies slightly — even with steady-state loads. Under transient load conditions, it can vary considerably. Research shows that small gaseous-fueled gen-sets exhibit frequency regulation inferior to diesel-fueled machines. Because some loads connected to the UPS will not tolerate a slew rate in excess of 0.5 Hz/sec, the magnitude of this compatibility issue becomes apparent when you consider gen-sets can have a slew rate of 10 Hz/sec to 15 Hz/sec (or more) for short periods of time. “Slew rate” describes the rate of change of frequency.

Therefore, a UPS may have difficulty in synchronizing its output to a gen-set, or may not be able to allow the maintenance bypass to operate if the gen-set frequency slew rate exceeds the allowed set point for this value in the UPS. If the UPS can't synchronize its bypass to the gen-set, the installation of an electronic governor on the gen-set or adjustment of the gen-set for slower response characteristics may help minimize the problem. You should be aware that whenever there is a sudden load application, or load rejection, the system is likely to initiate a “bypass disabled” alarm due to the change in frequency of the gen-set, which accompanies load changes.

Output voltage distortion. The degradation in voltage waveform quality caused by nonlinear loads associated with using UPSs can result in heating effects and shortened life in motors, misoperation of some equipment, and even activation of “bypass not available” alarms. The major difficulty is determining what level of voltage waveform quality you need for successful system operation. Then, you must specify a gen-set alternator that will provide this level of performance. The actual voltage distortion seen in a gen-set-powered circuit directly relates to the alternator sub-transient reactance.

Filtering equipment located at the UPS is useful in reducing total voltage distortion. But be aware of unexpected side effects. Filtering can have negative effects on gen-sets when the overall load level is very low. In these situations, the total power factor may become leading and the gen-set may be unable to regulate its voltage.

Transfer switch and power transfer issues

Another component commonly misapplied with power sources that feed UPS equipment is the automatic transfer switch. The need for oversized neutral requirements and the avoidance of nuisance tripping of the input breaker can cause problems. Oversized neutral capacity in a transfer switch is not commercially available, so transfer switches must be derated for nonlinear loads if the neutral must be of significantly greater capacity than the phase conductors.

Fast operation of transfer switches between live sources can result in nuisance breaker switching due to the inductive nature of the UPS load. Generally, a switching system that uses slow transfer (adjusted to about 30 cycles) will correct this potential problem. Another more expensive fix would be to use a closed transition transfer (paralleling the gen-set source with the utility source for a short time and then disconnecting the gen-set).

Suggested solutions

Implementation of the following recommendations will result in a much higher probability of success in UPS/gen-set installations:

Gen-sets providing power to UPS loads should include an AVR immune to misoperation due to waveform distortion. Three-phase sensing regulators and separately excited alternators are recommended, but not required, for proper system performance.

When available, the gen-set should have a sub-transient reactance of 15% or less based on the standby rating of the unit. Lower reactance yields better performance. But be careful! When the system is large, the short-circuit current available from the gen-set may be too large for the the switchgear you're providing — particularly in utility paralleling situations.

Specify UPS systems with input filters (especially with 6-pulse UPS units) to minimize voltage waveform distortion. Check the reverse reactive capability of the proposed gen-set to be sure the capacitive components of the filters will not disrupt the gen-set. A permanent reactive load bank will help the generator set maintain voltage regulation while the UPS is ramping on.

Gen-sets will usually need isochronous governors to provide 50 Hz or 60 Hz output at all load levels. This enables the UPS output to be more effectively synchronized to the generator power and help avoid activating “bypass disabled” alarms. It's possible to use mechanical governing successfully, but there may be problems with synchronizing the bypass of the UPS to the gen-set source as the load on the UPS changes.

Gen-set sizing should be based on the total load applied by the UPS under worst-case conditions. This means that even though the UPS normally operates at a relatively low-load level, the gen-set should be sized for full output of the UPS. You must also consider the battery recharge rate, UPS power factor, and efficiency at full load. You should use a system design limit of no more than about 10% to 15% total harmonic distortion (THD). Attempts to achieve a lower THD by oversizing the gen-set will result in a very large gen-set relative to the load.

Specify a program transition (“open” time of 0.5 sec or more) for transfer switches feeding UPS loads. A separate isolation transformer serving the UPS will be useful in avoiding operational problems, provide better power quality to the load, and prevent ground-fault sensing problems.

Working carefully with the gen-set and UPS supplier during the specification process will take care of many of these compatibility issues. If you take time to work them out, the result will be an emergency/standby power system that operates with your UPS with the highest degree of reliability.

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